In a computer disc drive, data is stored on discs in concentric tracks.
In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read or write operations. This is accomplished utilizing prerecorded servo information either on a dedicated servo disc or on sectors that are interspersed along a disc. During track following, the servo information sensed by the head is demodulated to generate a position error signal (PES) which provides an indication of the distance between the head and the track center. The PES is then converted into an actuator control signal, which is used to control an actuator that positions the head.
Historically, only one actuator, typically a voice coil motor (VCM), was used to position the head. Recently, micro-actuators have been proposed that would be used in combination with the VCM to position the head. Such micro-actuators generally have a better frequency response than the VCM. As such, they are better able to follow high frequency control signals. Thus, in disc drives with high storage capacities and densities, such dual-stage actuators are suitable for providing the required high-bandwidth and high-accuracy positioning. In some dual-actuator disc drives, the micro-actuator or second-stage actuator is a piezoelectric micro-actuator that uses piezoelectric elements made of a lead-zirconate-titanate material. Such a micro-actuator system can be referred to as a PZT system.
Ideally, the PZT system of a dual-actuator disc drive will maintain a constant gain during disc drive operation. In practice, however, the temperature of the disc drive fluctuates as a result of the changes in ambient temperature, etc., and therefore the gain of the PZT system, which is sensitive to variations in temperature, also fluctuates. In fact, PZT system gain variations as high as 20–30% have been found to occur during disc drive operation. Such gain variations can degrade the performance of the disc drive and, in some cases, cause intermittent stability problems in the servo loop.
One technique to adjust the micro-actuator gain involves the use of an off-line process to compute and store micro-actuator gain correction data in the disc drive. The data determined from this off-line process is subsequently used to adjust the micro-actuator gain during operation of the disc drive. In this context, an off-line process is one in which the micro-actuator gain correction factors are not determined continuously or in “real-time” during disc drive operation, but instead are determined from a calibration process that is carried out during manufacture of the disc drive. Such an off-line process is complex, time consuming, utilizes storage space in the disc drive and is relatively inaccurate.
Embodiments of the present invention provide solutions to these and other problems, and/or offer other advantages over the prior art.